The present invention relates generally to an electric relay system and more particularly, to a relay system for switching an A.C. power line utilizing a combination of an electromagnetic type relay having mechanical contacts, a bilateral gated semiconductor switch such as a triac having no mechanical contacts, and a control circuit for coordinating the switching actions of the relay and the triac.
Among presently available switching devices for making or breaking alternating current circuits are electromagnetic type relays having mechanical switching contracts which can be opened and closed under electromagnetic force, and semiconductor devices such as bilateral gated semiconductors having no mechanical witching contacts. The mechanical switching contacts in electromagnetic type relays tend to produce electric arcs when the contacts are opened. Therefore, great care must by expended when such relays are used in locations such as factories and warehouses having environments in which explosive or ignitable gases, liquids, dusts, or similar substances may be present. When a mechanical type relay is used an aircraft or ship, provision must also be made for preventing the generation by induction of interfering radio waves due to switching of the relay. An additional problem with the mechanical type of relay switching is that the switching contacts gradually wear away and sometimes are fused together.
On the other hand, the bilateral gated semiconductor switching device is disadvantageous in that it has non-negligible internal resistance which results in heat generation when in a conducting condition and requires a cooling fin of large size to dissipate the heat and prevent thermal breakdown. Thus, a relay of this type frequently is larger in size and greater in weight than the electromagnetic type relay for a given rating. Furthermore, the voltage drop across the device when the semiconductor relay is conducting is typically from 1.0 to 1.5V which is much greater than for a mechanical relay.